Editing Packet radio (section)

== Concepts ==
{{unreferenced section|date=February 2016}}

Packet radio can be differentiated from other digital radio [[Packet switching|switching]] schemes by the following attributes:

* Transmitted data is broken into packets, each of which contains a destination (and typically the source) address
* A transmitted message may be broken into a sequence of packets before transmission, which are then re-assembled into the original message upon reception
* Packets for multiple destinations can be transmitted on the same radio link in an [[Asynchronous communication|asynchronous]] fashion
* A packet may be addressed to all possible recipients rather than a specific one ([[Broadcasting (networking)|broadcast]])
* A packet may be stored and subsequently forwarded towards its destination by a network [[Node (networking)|node]]

This is very similar to how packets of data are transferred between nodes on the [[Internet]].

One of the first challenges faced by amateurs implementing packet radio is that almost all amateur radio equipment (and most surplus commercial/military equipment) has historically been designed to transmit voice, not data. Like any other digital communications system that uses analog media, packet radio systems require a modem. Since the radio equipment to be used with the modem was intended for voice, early amateur packet systems used [[AFSK]] modems that followed telephone standards (notably the [[Bell 202]] standard). While this approach worked, it was not optimal, because it used a 25&nbsp;kHz FM channel to transmit at 1,200 baud. When using a direct [[Frequency-shift keying|FSK]] modulation like G3RUH's packet radio modem, a 9,600 baud transmission is easily made in the same channel. In addition, the [[baseband]] characteristics of the audio channel provided by voice radios are often quite different from those of telephone audio channels. This led to the need in some cases to enable or disable pre-emphasis or de-emphasis circuits in the radios and/or modems.

Another problem faced by early "packeteers" was the issue of [[Asynchronous serial communication|asynchronous]] versus [[Comparison of synchronous and asynchronous signalling|synchronous]] data transfer. At the time, most personal computers had asynchronous [[RS-232]] serial ports for data communications between the computer and devices such as modems. The RS-232 standard specifies an asynchronous, start-stop mode of data transmission where data is sent in groups (characters) of 7 or 8 bits. Unfortunately, the simple AFSK modems typically used provide no timing signal to indicate the start of a packet [[Data frame|frame]]. That led to the need for a mechanism to enable the receiver to know when to start assembling each packet frame. The method used is called [[HDLC#Asynchronous framing|asynchronous framing]]. The receiver looks for the "frame boundary octet," then begins decoding the packet data that follows it. Another frame boundary octet marks the end of the packet frame.

A number of data "conversations" are possible on a single radio channel over a finite period.

A basic packet radio station consists of a [[computer]] or dumb terminal, a [[modem]], and a [[transceiver]] with an [[antenna (electronics)|antenna]]. Traditionally, the computer and modem are combined in one unit, the [[terminal node controller]] (TNC), with a [[dumb terminal]] (or [[terminal emulator]]) used to input and display data. Increasingly, [[personal computers]] are taking over the functions of the TNC, with the modem either a standalone unit or implemented entirely in [[software]]. Alternatively, multiple manufacturers (including Kenwood and Alinco) now market handheld or mobile radios with built-in TNCs, allowing connection directly to the [[serial port]] of a computer or terminal with no other equipment required. The computer is responsible for managing network connections, formatting data as AX.25 packets, and controlling the radio channel. Frequently it provides other functionality as well, such as a simple [[bulletin board system]] to accept messages while the operator is away.